1. Field of the Invention
This invention relates to an improved system for sensing engagement of a vehicle restraint with a transport truck or trailer which is to be restrained.
2. Prior Art
U.S. Pat. No. 4,127,856 Bickel describes the use of electrical switches to sense the engagement of the vehicle restraint with a bar mounted on the rear of a vehicle. The use of signals indicating engagement of the locking device is now well known in the art. For example, the Rite-Hite "Dok-Lok", Serco "VR" and Kelley "Truk Stop" and "Star" all have models that sense contact with a vehicle ICC bar and generate signals for use by personnel at the loading dock. These techniques are mentioned or described in many U.S. Patents including, U.S. Pat. Nos. 4,267,748; 4,282,621; 4,488,325; 4,553,895; 4,692,755; 4,695,216; 4,759,678; 4,830,563; 4,843,373; 4,938,647; and U.S. Pat. No. 4,946,330.
One characteristic of all the previous devices is that the electrical switches generate a signal by the presence or absence of physical contact with a moving part. Thus in each device a switch senses a change in the position of a component of the restraining device. The switch does not directly sense the presence or absence of the bar mounted on the rear of a vehicle. While such systems generally function well, and provide useful information to the control system, there are two major deficiencies in all of the previous systems. First, the loading dock area and the rear end of trailers can be very harsh environment as a result of rain, snow, ice, road salt and other materials present at a loading dock. The devices described above use mechanical limit switches which have moving parts which can become corroded or made inoperative by ice, dirt or other debris. Even if the limit switch is an inductive proximity switch, all of the switches described are activated by the motion of a mechanical component of the restraining device, and are therefore subject to malfunction or false signal if one of the moving components should become broken or stick from corrosion or other foreign matter.
For example, in the device proposed by U.S. Pat. No. 4,488,325 the sensor plate mounted on the hook shank trips a switch located in a recess below it and the entire sensor structure is exposed and subject to damage. Moreover, because the sensor plate has a pivot point within the shank length of the hook there is a lack of sensitivity at the pivot point. ICC bars exist in a number of different configurations and are typically deformed in use. Thus there is no assurance that a sensor plate of the type used in the '325 patent will provide a positive signal indicating engagement for the spectrum of ICC bars that exist.
In addition, the switch may be generate a false signal if the moving component is activated by foreign material, such as a broom handle or wood from a broken pallet, rather than the bar mounted on the rear of a vehicle.